It only works with supported carriers - but it can be a great way to keep connected when out of site of a tower.
T-Mobile has had WiFi calling on a few select phones since 2007, and Sprint has begun to offer it on a few phones as well.
Verizon and AT&T on the other hand have not enabled WiFi calling yet, probably because their cellular coverage areas are much more pervasive so that the need for WiFi calling support is a lot less urgent.
The catch with WiFi calling -- if you are using a limited data connection (like a MiFi) the data used to make a voice call will end up chewing up your data plan. Mobile users will need to be careful to make sure that their cheap T-Mobile phone doesn't chew through all their Verizon MiFi data!